The first sign, says Frank Bruno, that anything was wrong was when he stopped wanting to work out. From the age of 14 he had channelled all the energy a body his size generates into obsessive gym work. Even when his boxing career ended and his marriage fell apart and the TV offers dried up, he still managed to dredge up the motivation to keep himself fit. Then, at the beginning of 2003, he lost the will to exercise. "I just got fed up, man. I got pissed off. A lot of tension, a lot of different feelings going on through my head. I couldn't be bothered. I didn't give a shit. But it was the worst thing I could have done."

It was the beginning of a descent which, as everyone now knows, would end on September 22 of that year when Bruno was sectioned and held for 28 days in the Goodmayes psychiatric hospital in Essex.

Today he stands at a bar in Soho and looks a little sheepish, as if he hasn't quite decided how to present himself in light of all this. The old Bruno persona clearly won't do. Where once he was jocular, now everything about him tends towards the subdued; the black silk shirt and loose-fitting trousers that seek to downplay his size (they don't - he still fills the room like an obelisk); the excessive chivalry; the handshake so forceless it's like being touched by a ghost. Two weeks ago, the News of the World splashed with details of Bruno's former cocaine abuse, the revelatory selling point of his memoir, Fighting Back. Although he doesn't take drugs any more, the battle to be happy, says Bruno, is not over yet.

Looking back it's a wonder anyone was surprised when he was diagnosed with bipolar disorder. His public appearances had for a long time been characterised by a manic repetitiveness (particularly of the word "wicked") and a blinding good humour that looks, in hindsight, like the exaggerated high of a very big low. He was a national hero less for being a good boxer than a good sport, a man of such boundless equanimity that TV producers presented his manner as a side-effect of too many blows to the head. Then, as now, one wondered the extent to which he was in control of the joke.

"Mustn't grumble," says Bruno of the way things have turned out, and his eyes slide sideways. Today the only drug he takes is Lithium which, he says, "the doctor says I have to take, for my moods and different things like that". He is exercising again, so much so that friends have called him a "prisoner of the gym," not least because he lives in an apartment in the health farm Champneys, which is owned by a friend. He finds it preferable to being alone in a house. "I like going to the gym every day. Some people like going to the pub; I enjoy going to the gym. It's a big priority. Sometimes I get carried away."

It was loneliness, says Bruno, which was his undoing, even though his career had been one long lesson in it. "Boxing is the toughest and loneliest sport in the world. You've got all the fans, lots of hangers-on jumping up and shouting different words. But when you actually go in the ring, it's a very lonely and scary place. It's just you and the other guy."

In Bruno's case the "other guy" tended to win, or at least that was the impression one always had. In fact, he won more fights than he lost over the course of his 15-year career. But a lot of his victories were in the early years, in his native south London, when the stakes were quite low. He never beat the top guys, not Mike Tyson nor Lennox Lewis, and although he won the world heavyweight title in 1995 against Oliver McCall, he lost it six months later to Tyson, in the last fight of his career.

I ask why he never took part in the verbal abuse that boxers traditionally exchange at the weigh-in. Bruno has described himself as an angry young man, frustrated by his poor performance at school and by his home life, where throughout his early teens his father was bed-ridden with cancer. "It took him five years to die; I wish he'd gone earlier rather than suffering." But by the time he became a professional boxer, Bruno's bad attitude seemed to have evaporated, which was not, in his line of work, necessarily considered a good thing.

"I'm not being funny," he says, "but I mean ... I'm not into all them slagging matches. It helps sell tickets, it helps the promoter, but it consumes too much energy. If I can avoid it, I will."

Did he feel intimidated when members of Tyson's entourage drew a finger across their throats at him as he walked to the ring? "I would try to block it out. Some of them were coming out with a lot of shit; I didn't get involved."

Bruno says he likes Tyson, who wrote him an open letter of support after his breakdown. I'm surprised by this, given the pummelling Tyson gave him in the last title fight. But I suppose if there is anyone in boxing who can make Bruno's problems look small-scale, it's Tyson. Everything Bruno has done, or is alleged to have done, Tyson has done bigger. "He's a dangerous guy; a dangerous human being. But deep down he's nice and I like him. Yeah, I like him. I like him. I like him." He pauses. "I'm not gay or nothing like that; I don't like him in that sense. But he's an alright guy."

For the first time in the interview he laughs that extraordinary laugh of his that seems to provide its own echo. Then he stops abruptly and clarifies his position on gay rights. "I ain't got no right to judge someone. I haven't got anything against lesbians, neither. We live in a world where they've got as much right as anybody else. I know a lot of gay people in the pantomime business and they're very nice. Gay people and lesbians, you know; whatever."

While Bruno's reputation has survived the drugs scandal and his breakdown, there are still lingering doubts about what happened during his marriage to Laura. They met at an ice rink in 1980 and married 10 years later. Then in 1997 Laura was reported to have obtained a court order banning her husband from "assaulting, molesting or harassing" - allegations he always denied, and for years the couple yo-yoed between separation and reconciliation. They have three children and the terms of the divorce, which was finally granted in 2001, prevent him from discussing it in detail. Among the grounds for being sectioned are that one is a danger to oneself or to others; Bruno insists his relate only to the former. But he admits that he must have been difficult to live with. It was when Laura left for the last time that things really started to go downhill.

"Paranoia," he says. "I'd listen to three radio stations at the same time. Different channels coming in. I got confused and snappy and impatient. I couldn't, couldn't, couldn't function. Losing my wife, seeing my kids less regularly, not eating properly, staying up late, up and down the motorway like a yo-yo. [He was making money doing bits and pieces of DJing.] Living by myself, getting uptight, wound up, over stupid little things. If you're not balanced, your mind's not balanced ... my fuse went. The lights were on, in the house, but there were no bulbs in there."

The usual consolations didn't work. Where once he would relax by "going into my music room and making a couple of tapes", he started to "hate music". He invited some young boxers, one of them Wayne Rooney's brother, from Liverpool to stay with him in the house, under the auspices of offering them training. But he wasn't up to it and although he can't quite remember how, a load of freeloaders moved in. "Not the Liverpool kids, but others who were dodgy and shouldn't have been there."

Things got worse when Bruno's great friend and mentor, his former trainer George Francis, killed himself after his wife and son died in the same year. "George would've said to me keep training, keep looking after yourself, face-wise, emotionally-wise, inner-wise. Spiritually. He looked after himself, George. But unfortunately the pressure got to him and he just said eff it, I'm going."

Was Bruno suicidal? He sucks his orange juice noisily through his straw. "Nah, I'm not that brave. I'm not that clever to do a suicide. I wouldn't know how to tie a rope in the first place, you know what I mean? I felt down, and I felt upset with myself. But I never felt suicidal."

He thinks it was his daughter who had him sectioned. "A combination of my daughter, my friend and some nurses who came round to check on me. I was being horrible to them. I didn't mean to be; but I wanted them to leave me alone." Of course, he says, he doesn't hold it against her. He takes out his mobile phone and shows me an "I love you dad" text message she sent him last week.

During the first few days at Goodmayes he fought against the authorities. But then, he says, he went to a couple of therapy sessions and started to find it interesting. He didn't see the Sun's headline breaking the story - "Bonkers Bruno" - but someone told him about it. "I can't knock them," he says. "They helped me fight, helped me be famous; if it weren't for the press I wouldn't be here now, especially the Sun. I can't really get involved. They can call me whatever they want."

This suggests a keener self-awareness than Bruno has been given credit for, particularly during his heyday in the 1980s when his television appearances as the thicko boxer with the catchphrase - "know what I mean, 'arry" - earned him accusations of Uncle Tom-ism from, among others, Lennox Lewis. Mention of this still gets Bruno agitated. "A lot of people were very jealous. When I came into boxing, I brought it to the next level with adverts and doing pantomime and people just got jealous of me doing that. And they started calling me all sorts of different names. But if they'd had the opportunity to do it, they would have. You know Chris Eubank told me he would have loved to have got into pantomime."

So why did Bruno succeed where they failed? "I don't know. I had a good manager in Terry Lawless. I had good backing people, like Jarvis Astaire. They knew how to market me in that way. But it's jealousy."

But they marketed him as a caricature. "It was a caricature, that I was ducking and diving. But people just got jealous. Concoctions and rumours and confusion and shit." He looks cross and confused.

Has he spoken to Lewis recently? "I haven't seen Lennox Lewis for ages. If he would come down off his pedestal and chill out a little bit, yeah, I'd talk to him. But if he don't want to talk to me I ain't going to lose no sleep."

Bruno is comfortably enough off not to have to think about working. But he is aware of what idleness does to him and he would like to get back into some kind of performing. "Hopefully pantomime," he says, as if resigning himself to an unpleasant fate, "or a TV soap like East- Enders. Just keep pecking along like everybody else does." If he misses the adrenaline of the ring he just goes round to his mate's house and lets his rottweiler "chase me around for a bit. Ha ha."

Now 43, he has thought about training young boxers again, but he says it requires too much "emotional involvement". He knows his limitations. "Mind you," he says, "I wouldn't mind getting emotionally involved with a woman." He is starting to feel strong enough to go out again, to nightclubs. "I don't need anything to get me in the mood. Once I'm in a good mood I don't need anything to make me enjoy myself. I'm a nutter, naturally, by itself. I can be the life and soul."

Despite everything, if he hadn't gone into boxing he believes his life would have been infinitely worse. "I'd be out doing odd jobs, or sweeping the road, or be the metal polisher or on a building site or doing some criminal activity. My head has got regrets, but I haven't. I've done pretty well for myself and met some nice people and done some nice things. Regrets, no."

We go outside for the photos and Bruno is recognised by everyone who passes. "All right, boss?" he calls to a staring traffic warden. A chef from the Italian restaurant next door yells, "Do you want to fight me?" and Bruno flicks him a savage look. "Chloe will fight you," he says, indicating the PR woman. He poses in a doorway and says, quietly, "Wicked, wicked, wicked, wicked."

And then the photographer tries to show him his image as it appears on the screen of the digital camera. Bruno shrinks from it, like Dracula before a mirror. "I don't want to see it," he cries. "I don't want to look at myself ".

· Frank Fighting Back is published by Yellow Jersey Press, £18.99